Add a unit test for DuplicateCNAME (#329)
Add a unit test for DuplicateCNAME (#329)

This fix adds a unit test for DuplicateCNAME to have
a basic coverage for dnsutil.

Signed-off-by: Yong Tang <765086fe2e0c1f980161f127fec596800f327f62@outlook.com>
ii/coredns,coredns/coredns,jremond/coredns,ii/coredns,johnbelamaric/coredns,varyoo/coredns,chrisohaver/coredns,miekg/coredns,miekg/coredns,jremond/coredns,johnbelamaric/coredns,yongtang/coredns